The field of skin contact devices in general and transdermal devices in particular has undergone significant development in the last several years. With respect to the application of film layers broadly, that is with or without an active agent medicament therewith, there has been a general development based upon the discovery in the early 1970's, contrary to previous beliefs, that the healing of wounds occurs significantly faster and with less scar formation when it takes place under a film which is capable of maintaining a well-balanced moist environment, such as that which naturally occurs under a blister.
In view of this understanding, a number of different medical and surgical dressings were developed, leading most recently to the devices which are shown and disclosed in U.S. Pat, No. 4,915,102. In the background section of that patent, there is a detailed discussion of a number of other devices which had been developed prior to that time, and whose function was to provide various types of wound dressings taking advantage of this discovery.
With respect to the specific field of transdermal application, in addition to the fact that the above-mentioned U.S. Pat. No. 4,915,102 also specifically discloses devices for applying both a film layer and an active agent carrier, a number of other such devices have also been developed.
Thus, predecessors to the devices shown in U.S. Pat. No. 4,915,102 include devices such as those shown in U.S. Pat. No. 4,573,996. These devices each include a reservoir of an active agent, a release mechanism for releasing the active agent through the skin of the patient, and an outer layer which is variously described as being "occlusive." Thus, in the particular environment in question, these layers are required to be impermeable to the active agent itself, so as to prevent its escape, and are also impermeable to the passage of water vapor therethrough.
Another wound dressing is shown in U.S. Pat. No. 4,499,896. In this patent a wound dressing is disclosed, which includes a first layer 2, which is to be placed over the wound and which is impermeable to liquid water, but which has a moisture-vapor permeability of at 300 g/m.sup.2 for 24 hours at 80% relative humidity differential at 40.degree. C., and which includes perforations 3 for passing wound exudate through the first layer, and which also includes an imperforate second layer 5 which is impermeable to liquid water and has a moisture-vapor permeability of at least 600 g/m.sup.2 for 24 hours at 80% relative humidity differential at 40.degree. C. A reservoir is thus formed between the two layers to which wound exudate passes by way of the perforations of the first layer, and which then slowly evaporates to the second layer. The reservoir itself can also include a medicament.
Also, U.S. Pat. No. 4,787,888 discloses a drug delivery device which, in this case, employs a piezoelectric polymer element coupled with a sonic generator for supplying energy to produce sonic vibrations to drive the medicament into the skin. In this device the drug matrix layer 32 is located between the piezoelectric polymeric body and the skin, and the former includes polymer films 16 and 18, which are superimposed by a flexible film cover element 20. FIGS. 1 and 3 of this patent appear to illustrate the central layers 54 and 55 in bimorphs as having wide, perpendicular slits having an opening between the drug layer and the flexible cover layer. There is no discussion, however, in the specification regarding the presence of any such slits, let alone their purpose.
Column 3, lines 53-58 of this patent, in fact, refer to the assembly from "plural components." In any event, the piezoelectric material disclosed in this patent is PVDF, which is moisture-impermeable and, since the overall piezoelectric polymer layer includes layer 20, even the slits shown therein do not extend to the outer surface, i.e., they are covered by another water vapor-impermeable layer.
Problems have thus continued with a number of these transdermal devices and the like in maintaining skin contact and flawlessly performing the function thereof, particularly for extended periods of time. Thus, the principal objects of delivering the drug at a constant rate by maintaining complete contact with the skin over long periods of time have continued to require improvements. One factor which exacerbates this problem is the build-up of moisture within these devices. Another such factor is the relative stiffness of certain of these devices, which facilitates their eventual unintended removal from the surface of the skin.